Cephalotes setulifer is an obligate inhabitant of the ant-plant Cordia alliodora (Carroll 1983, Longino and Hanson 1995, Longino 1996). The first published record of a nest was from C. alliodora (Menozzi 1927), as was the second (Wheeler 1942) and the third (Kempf 1958). Wheeler considered the species to be a facultative inquiline, and assumed that it could be found nesting elsewhere. Kempf was more circumspect, stating "We need more factual evidence in order to decide whether setulifer is an obligatory or only a facultative inquiline of this ant plant." Andrade and Baroni Urbani (1999) follow Kempf in this regard. Carroll (1983) was the first to report on extensive collections from C. alliodora. He found C. setulifer to be a common and widespread inhabitant of C. alliodora, and he suggested that C. setulifer was an obligate plant-ant. In my experience, C. setulifer is perhaps the most ubiquitous inhabitant of C. alliodora in Costa Rica, occurring in most trees in the various populations I have surveyed. Despite extensive faunal surveys throughout Costa Rica, I have never found C. setulifer nesting anywhere other than in C. alliodora. C. setulifer is clearly an obligate plant-ant in C. alliodora.

Domatia inhabited by C. setulifer are easy to identify because the head of a soldier appears as a perfectly circular brown dot between the projecting stems. Poking this dot with a grass stem will cause the soldier to retreat into the domatium, but the soldier will quickly return when the stem is removed.

This species demonstrates that not all host specialists are "dominant" ants in the sense of exclusively occupying most of a hostplant and having large colonies. Cephalotes setulifer is an inconspicuous and timid ant, capable of coexisting in the same tree with any of the dominant ants that inhabit C. alliodora: Azteca pittieri complex, Azteca JTL-003, Azteca beltii, and Pseudomyrmex viduus. Hence, it may best be called an obligate inquiline.

The species appears to be monogynous. I have observed hundreds of nodes with founding queens, and all have been solitary.

Longino, J. T. 1996. Taxonomic characterization of some live-stem inhabiting Azteca (Hymenoptera: Formicidae) in Costa Rica, with special reference to the ants of Cordia (Boraginaceae) and Triplaris (Polygonaceae). Journal of Hymenoptera Research 5:131-156.